Polyarylene sulfide resin (PAS resin) is a thermoplastic resin possessing partial thermosetting properties and exhibiting superior chemical resistance, mechanical characteristics, heat resistance, and the like over a wide temperature range. This resin is known especially as an engineering resin possessing high rigidity and is useful as a heat resistance material for various parts of electronic or electric appliances.
At present, a batch process is mainly used as the commercial manufacturing method for polyarylene sulfide (PAS). As is well known a continuous process is superior with respect to energy efficiency, facility cost, and the like than this batch manufacturing method.
In the U.S. Pat. No. 4,056,515 is disclosed a method for producing PAS using a tank where unreacted p-dichlorobenzene (PDCB), water, and NMP are evaporated after polymerization, PAS is concentrated, and the liquid surface level is controlled using liquid surface level meter(s) which detect the both positions of the liquid phase and the vapor phase by controlling the flow-rate.
However, PAS particles produced by this process contain sodium chloride which is a by-product of the polymerization reaction in the amount of several thousands ppmw. A treatment such as washing with water cannot sufficiently remove the sodium chloride, that is, the resultant PAS still contain more than 200 ppm sodium chloride. The product has problems such as inadequate electrical insulation and corrosion when utilized to manufacture parts in electrical or electronic appliances.
In addition, a method for recovering granular PAS is disclosed in the U.S. Pat No. 4,415,729. This method consists of the following steps: an appropriate amount of separating reagent such as water is added to the reaction mixture which contains molten PAS, and granular PAS is phase-separated from polar solvent by lowering the temperature of the reaction mixture, and is recovered, collecting the granular PAS as described above as slurry which consists of said PAS, NMP, water, etc., separating PAS from the slurry, and washing PAS separated with water.
This method of recovery, however, has a problem that by-products such as lithium chloride formed during the polymerization step tend to remain in PAS, because said PAS is separated from polar solvent after solidifying PAS from the liquid state to granules. Therefore, there were problems that electrically insulation decreases by salts such as lithium chloride contained in PAS, and corrosion is caused easily in an electric circuit (metallic conductor) when PAS is applied to the electric/electronic field which is the main application field of PAS
The purpose of this invention is to address the above-mentioned problems and to provide a method for continuously producing PAS suitable for molding and injection molding the seat, the film the fiber, etc. that has a high molecular weight, and contains only a small amount of by-products such as lithium chloride (occasionally called "lithium salt" generically), efficiently and economically.